Conventional ultrasound scanners are capable of operating in different imaging modes, such as B mode and color flow mode. In the B mode, two-dimensional images can be generated in which the brightness of display pixels is based on the value or amplitude of respective acoustic samples representing the returned echo signal.
In a conventional ultrasound imaging system (shown in FIG. 1), an ultrasound transducer array 2 is activated to transmit a series of multi-cycle (typically 4 to 8 cycles) tone bursts which are focused at the same transmit focal position with the same transmit characteristics. These tone bursts are fired at a pulse repetition frequency (PRF). The PRF is typically in the kilohertz range. A series of transmit firings focused at the same transmit focal position are referred to as a "packet". Each transmit beam propagates through the object being scanned and is reflected by ultrasound scatterers in the object.
After each transmit firing, the echo signals detected by the transducer array elements are fed to respective receive channels of the beamformer 4. The receive beamformer tracks echoes under the direction of a master controller (not shown in FIG. 1). The receive beamformer imparts the proper receive focus time delays to the received echo signal and sums them to provide an echo signal which accurately indicates the total ultrasonic energy reflected from a succession of ranges corresponding to a particular transmit focal position. The beamformer also transforms the RF signal into its I/Q components by means of Hilbert bandpass filtering. The I/Q components are then summed in a receive summer (not shown) for each transmit firing. Hilbert bandpass filtering can alternatively be performed after beam summation.
The output of the beamformer 4 is shifted in frequency by a demodulator 6. One way of achieving this is to multiply the input signal by a complex sinusoidal e.sup.i2.pi..function.dt, where .function..sub.d is the frequency shift required. The downshifted I/Q components are then sent to a B-mode processor 8, which incorporates an envelope detector 10 for forming the envelope of the beamsummed receive signal by computing the quantity (I.sup.2 +Q.sup.2).sup.1/2. The envelope of the signal undergoes some additional B-mode processing, such as logarithmic compression (block 12 in FIG. 1), to form display data which is output to the scan converter 14.
In general, the display data is converted by the scan converter 14 into X-Y format for video display. The scan-converted frames are passed to a video processor 16, which maps the video data to a gray scale or mapping for video display. The gray scale image frames are then sent to the video monitor 18 for display.
The images displayed by the video monitor 18 are produced from an image frame of data in which each datum indicates the intensity or brightness of a respective pixel in the display. An image frame may, e.g., comprise a 256.times.256 data array in which each intensity datum is an 8-bit binary number that indicates pixel brightness. The brightness of each pixel on the display monitor 18 is continuously refreshed by reading the value of its corresponding element in the data array in a well-known manner. Each pixel has an intensity value which is a function of the backscatter cross section of a respective sample volume in response to interrogating ultrasonic pulses and the gray map employed.
A conventional ultrasound imaging system typically employs a variety of gray maps, which are simple transfer functions of the raw acoustic sample data to display gray values. Multiple gray maps are supported so that different maps may be used depending on the raw data. For example, if a given application tends to generate mainly low-level raw data, then a gray map which dedicates more gray-scale values to low-level raw data values is desired since it improves the contrast across this region. Therefore, it is typical to default to a different gray map depending on the application. However, this is not always effective since the user can scan any anatomy in any application, raw data varies from patient to patient, and the raw data depends on other system settings such as dynamic range. Due to these factors, the gray maps tend to be conservative with respect to how many gray-scale values are dedicated to the anticipated primary data range. Thus, there is a need to improve the contrast of the displayed pixel data by providing a means for gray mapping which is not based on assumptions about the raw acoustic sample data.